1. Field of the Invention
The present invention relates generally to a valve timing controller for use with an internal combustion engine and, more particularly, to the valve timing controller for varying the valve timing at which various valves such as intake and exhaust valves are selectively opened and closed, or the lift of those valves, in dependence on a rotational phase change or an axial movement of a camshaft.
2. Description of Related Art
In general, the internal combustion engine is equipped with a valve system for selectively opening and closing valves (intake valves and exhaust valves) at a predetermined timing synchronized with rotation of the crankshaft. The optimum valve timing at which the specific valves are opened or closed generally varies according to the engine operating condition such as, for example, the engine load, engine speed and the like. Some of the conventional engines have recently come to be equipped with such a valve timing controller that varies the valve timing according to the engine operating condition.
Japanese Laid-Open Patent Publication (unexamined) No. 4-272411, based on U.S. Pat. No. 5,031,585, discloses a valve timing controller having a phase change device, interposed between a pulley driven by a crankshaft and a camshaft driven by the pulley, for effecting a phase change between the camshaft and the pulley (camshaft). According to this publication, the valve timing is varied by changing the phase of the camshaft relative to the pulley, i.e., by advancing or retarding the rotation of the former relative to that of the latter according to the engine operating condition.
The valve timing controller disclosed in the Japanese Laid-Open Patent Publication No. 4-272411 comprises a spring, interposed between the camshaft and the pulley, for biasing the camshaft so as to return it to its reference position, and a drum for advancing or retarding the rotation of the camshaft against the biasing force of the spring upon application of a resisting force thereto. The resisting force is provided by an electromagnetic brake mounted in juxtaposition with a brake disc surface formed on the drum. The amount of advance or retardation of the camshaft is varied by controlling the magnitude of the resisting force of the brake applied to the drum. In this case, the amount of advance or retardation is maintained when the biasing force of the spring and the resisting force of the brake are well balanced.
This kind of conventional valve timing controller, however, has the disadvantage of bringing about an increase in electric power consumption because power supply to the brake is always required when the camshaft is maintained at an advanced or retarded position. Also, when the camshaft is at such a position, the resisting force of the brake is always applied to the drum and, hence, the drum or brake considerably wears, thus lowering the durability.
Furthermore, because the wear of the drum or brake changes the resisting characteristic of the brake, it is necessary to feed-back control the resisting force of the brake in order to maintain the amount of advance or retardation of the camshaft to a predetermined one, thus complicating control mechanisms. Also, when the camshaft is at the advanced or retarded position, the resisting force is always applied to the camshaft, resulting in an increase in power loss of the engine.